1. Field of the Invention
The present invention relates to a semiconductor device. In particular, the present invention relates to a semiconductor device including a photoelectric conversion element. In addition, the present invention relates to an electronic device using the semiconductor device.
2. Description of the Related Art
A number of photoelectric conversion devices generally used for sensing an electromagnetic wave are known, and for example, a photoelectric conversion device having sensitivity in ultraviolet rays to infrared rays is collectively referred to as an optical sensor. An optical sensor having sensitivity in a visible light region with a wavelength of 400 to 700 nm is particularly referred to as a visible light sensor, and a large number of visible light sensors are used for devices which need illuminance adjustment, on/off control, or the like depending on human living environment.
In particular, in a display device, brightness in the periphery of the display device is detected to adjust display luminance thereof. This is because unnecessary electric power can be reduced by detecting peripheral brightness and obtaining appropriate display luminance. For example, such an optical sensor for adjusting luminance is used for a mobile phone and a personal computer.
In addition, as well as peripheral brightness, luminance of a backlight of a display device, in particular, a liquid crystal display device is also detected by an optical sensor to adjust luminance of a display screen.
In such an optical sensor, a photodiode is used for a sensing part and output current of the photodiode is amplified by an amplifier circuit. As such an amplifier circuit, a current mirror circuit is used, for example (e.g., see Patent Document 1: Japanese Patent No. 3444093).
In addition, a semiconductor optical sensor device in which an illuminance measurement range is extended is disclosed in Patent Document 2 (Patent Document 2: Japanese Published Patent Application No. 2006-86425). The semiconductor optical sensor device disclosed in Patent Document 2 is provided with two photodiodes for low illuminance and high illuminance. A switch is controlled by comparing output and reference voltage so as to select which of the photodiodes is to be used.